As one type of mass spectrometers, a quadrupole mass spectrometer has been known in which a quadrupole mass filter is used as a mass analyzer which separates ions depending on their mass. FIG. 6 is a schematic configuration diagram of a typical quadrupole mass spectrometer, which mainly shows an ion optical system of the mass spectrometer.
A sample molecule is ionized in an ion source 1. Generated ions are converged (also accelerated in some cases) by an ion transport optical system 2, such as an ion lens, and introduced into a space in the longitudinal axis direction of a quadrupole mass filter 3. The quadrupole mass filter 3 includes four rod electrodes arranged in such positions as to be parallel to one another around an ion optical axis C (In FIG. 6, only two electrodes are shown). Voltages obtained by adding direct-current voltages±U and radio-frequency voltages±V·cos ωt are applied to those rod electrodes, respectively. Depending on the applied voltages, only the ions having a specified mass selectively pass through the space in the longitudinal axis direction of the quadrupole mass filter 3 whereas other ions are diffused along the way. A detector 4 outputs an electrical signal corresponding to the amount of ions having passed through the quadrupole mass filter 3.
As mentioned previously, the mass of ions passing through the quadrupole mass filter 3 varies depending on the voltages applied to the rod electrodes. Accordingly, the mass of ions arriving at the detector 4 can be scanned over a predetermined mass range by changing the applied voltages. This is a scanning measurement in a quadrupole mass spectrometer. In the case of a gas chromatograph/mass spectrometer (GC/MS) or a liquid chromatograph/mass spectrometer (LC/MS), for example, where the components of a sample introduced in the mass spectrometer change with time, various components that sequentially appear can be detected almost continuously by repeating the scanning measurement. FIG. 7 schematically shows a change in the mass of ions arriving at the detector 4 while the scanning measurement is repeated.
In such a scanning measurement, an increase in the scanning rate, i.e. the amount of change in the mass per unit time, shortens the time required for a single mass-scanning cycle. This means that the frequency of the scanning measurement implemented in a certain predetermined time can be increased in the repetitive scanning measurement. Accordingly, in GC/MS or LC/MS, time resolution is improved as the scanning rate is increased, thereby avoiding a failure in the detection of a component which appears only for a short time. Furthermore, in these days, many efforts have been made to speed up the component separation in an LC, for example, in order to improve the throughput of an analysis. On such occasion, it is important to improve the time resolution of mass spectrometry. For this reason, a further increase in the scanning rate is presently required.
However, the increase in the scanning rate causes the following problems: Consider the case where the necessary time for a certain ion to pass through the space in the longitudinal axis direction of the quadrupole mass filter 3 is “t”. The necessary time t depends on the kinetic energy of each ion at the time when the ions arrive at the entrance of the quadrupole mass filter 3. FIG. 8 shows a relationship between time and voltage applied to the quadrupole mass filter 3. During the scanning measurement, the voltage applied to the quadrupole mass filter 3 is scanned so that it continuously changes. Accordingly, as shown in FIG. 8, the applied voltage changes even during a period of time when a certain ion is passing through the space in the longitudinal axis direction of the quadrupole mass filter 3. The higher the scanning rate is, the larger the amount of change ΔV in the applied voltage within the time t is.
The aforementioned change in the applied voltage means that the condition under which a certain ion passes through the quadrupole mass filter 3 (the mass of the ion passable therethrough) changes while the certain ion is passing therethrough. If the scanning rate is sufficiently low and the amount of voltage change ΔV is negligibly small, the aforementioned problem does not substantially occur. However, if the amount of voltage change ΔV is non-negligibly increased by increasing the scanning rate, there is the possibility that a part of the target ions cannot pass through the quadrupole mass filter 3. This causes a decrease in the amount of ions arriving at the detector 4, thereby deteriorating the detection sensitivity.
FIG. 9 shows a mass spectrum measured with a conventional quadrupole mass spectrometer. The upper row shows a mass spectrum obtained when the scanning rate was set at 125 [Da/sec], while the lower row shows a mass spectrum obtained when the scanning rate was set at 7500 [Da/sec]. In either row, the respective peak points are located at the mass of m/z 168.10, 256.15, 344.20, 520.35, 740.45, 872.55, 1048.65 and 1268.75, starting from the left of FIG. 9. It can be understood that the higher the scanning rate is, the narrower a peak width is, which means that the mass resolution is higher. On the other hand, it can also be understood that the higher the scanning rate is, the lower a peak height is, which means that the detection sensitivity is lower. This phenomenon is particularly remarkable when the mass is high.
In order to cope with the previously described problems, in the mass spectrometer disclosed in Patent Document 1, a bias voltage, which is applied to the respective rod electrodes of the quadrupole mass filter 3 separately from the voltages applied for ion separation, is changed so as to diminish the influence of the change in the scanning voltage on the ions passing through the quadrupole mass filter 3. If the bias voltage is changed, the kinetic energy of ions introduced into the quadrupole filter 3 varies. Accordingly, when the scanning rate is high, the bias voltage is changed in such a manner that the kinetic energy of ions introduced into the quadrupole mass filter 3 is increased. By this method, when the scanning rate is high, the passing time t of the ions becomes relatively short, making the amount of voltage change ΔV be relatively small and thereby avoiding deterioration in the detection sensitivity.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-25498    Patent Document 2: Japanese Unexamined Patent Application Publication No. H8-102283    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2005-259616